How to build climbing endurance off-the-wall as a beginner climber?

Exactly as the title says. I have no gym, and only get to climb a couple times a month if im lucky. But I notice I make it up one route, a 5.9, then cannot make it up another 5.9 without resting on the rope after every clip. I know my technique needs a ton of imporvment, and that this will help with endurance, but I cannot constantly climb to improve my technique.

What can I do at home to improve? Lots of pull-ups? 

I would look into some hangboarding endurance workouts if I couldn't climb more than once a month. Here's one article, but I'm sure there's more info out there on it. You will probably benefit the most from more time actually climbing if you can find a way to make it work (build a home board, find a local board that you can climb on, etc.). You can even find buildering problems that you can traverse on to get some endurance.

It appears that 5.9 is too challenging for you right now. When you do get to climb, it will probably be more beneficial to choose easier options to get more high quality movement opportunities. You might get better longer term endurance gains by using every hold, aka "rainbow" climbing, and doing more successful moves in a session.

Do you do any other sort of endurance that will improve your overall all cardiovascular fitness? Such as running, biking or swimming.....

due to time limitations, i have periods when i can't make it to the gym for a long time. in those periods, i do endurance-focused workouts that my friend recommended to me, either using a hangboard or a pull up bar, and a chair. place the chair out in front of you, not under you. the farther out, the harder the workout, so you can work your way out. there are two variations that i do, and i have found both very helpful:

1) hang in a 90 degree lockoff for 5 seconds with feet off, then continue to hang however you want without letting go but with your feet now on the chair, for 30 seconds. so 1 set is 5 seconds of intensity, 30 seconds of "climbing rest". repeat for 15-25 sets, never completely letting go of the bar. i wear a chalk bag and alternate hands to chalk up during the rests.

2) same idea as the above but 1 set is 3 seconds of a dead hang (not lockoff) and 7 seconds of "climbing rest" with feet on the chair. there is not enough time to chalk up here.

option 1 feels like more like pure endurance, and option 2 feels more like power endurance. option 2 actually improved my technique because the intense periods come very quickly and end quickly and as you're moving your legs on and off the chair, you swing. swinging is bad and makes it harder to hold on, and it gave me a good appreciation for how to grab the bar and how to control my body in these constant transitions, and this translated to climbing for me, grabbing holds well and being aware of body position during loading.

there was a time i did each of these once a week for several weeks in a row and i got good results for the short time invested. good luck!

I'm pretty new to climbing, so this may not be the best advice, but here's what I've learned as a weightlifter, what I've found over the past few months of climbing indoors, and other tips I've heard:

If you're climbing one 5.9 and then struggling on the next, the limiting factor is often forearm endurance, climbing efficiency, or your recovery between routes. Think of it similarly to recovering between challenging sets while lifting weights. After a hard climb, your muscles are fatigued, so giving them enough time to recover before your next attempt helps you perform much better! If you have access to a pull-up bar or hangboard, practicing easy hangs with short rest periods between them is a good way to build forearm endurance, but this is not the only factor. Focus on endurance rather than maximum strength, especially if you're new to hangboarding. General strength training has been a key factor for me, including pull-ups, chin-ups, and scapular pull-ups, and don't neglect your core. Exercises like planks, side planks, dead bugs, L sits, and hanging knee raises have helped me a lot. Strong legs are equally important because they should be doing most of the work while you climb. For this, I'd recommend Bulgarian split squats, step-ups, and Romanian deadlifts are all great exercises to build leg strength. When you do get the chance to climb, spend time mastering easier routes first. I'm still making sure I've got the 5.8/5.9s dialed in before I move onto the 5.10s, and that's all indoors! Climb them multiple times while focusing on good footwork, keeping your hips close to the wall, and using your legs to drive your movement instead of relying solely on your arms.